Aircraft gas turbine engines are often exposed to extreme conditions during operation that result in degradation or compromise of structures therein, resulting in required maintenance or replacement of various parts of the engines. To impede degradation of structures in the engines, various coatings may be formed on the structures. For example, an environmental barrier coating (EBC) may be formed on various structures to protect the structures from oxidation and corrosion due to exposure to oxygen and water vapor, as well as other airborne contaminants. A thermal barrier coating (TBC) may also be formed over the structures in the engine, independent from the EBC, to effectively insulate and minimize thermal impact on the structures in the engine due to temperature cycling.
TBCs may be formed through a physical vapor deposition (PVD) process to develop a columnar microstructure of the TBC, with gaps defined between columnar grains in the TBC. Further, TBC materials are generally chosen from oxide ceramics having low thermal conductivity, with zirconium oxide commonly employed.
The columnar microstructure of the TBCs enables the TBCs to provide effective thermal insulation to underlying structures while resisting cracking or delamination during thermal cycling. In particular, gaps between individual columnar grains in the microstructure allow the TBC to expand and contract without developing stresses that could lead to spalling. The TBCs that have the columnar microstructure may be compromised under various circumstances. For example, TBC degradation may result from ingestion of airborne particles into the engines during operation. The airborne particles, commonly referred to as calcia-mangesia-alumina-silicate (CMAS), can melt at high operating temperatures of the engines and infiltrate the gaps between the columnar grains. Upon cooling, the infiltrated CMAS solidifies and thusly increases stiffness of the coating, leading to thermo-mechanical degradation of the TBCs.
Various approaches have been investigated to minimize degradation of the TBCs due to CMAS infiltration and thermo-mechanical degradation associated therewith, although a clear-cut solution has yet to be identified. One common approach is to introduce a TBC material that reacts with CMAS to produce a stable, high melting temperature compound that would also block open gaps on the TBC surface and prevent infiltration of CMAS into the gaps; however, this approach may result in inconsistent surface properties of the TBC. In fact, adequately inhibiting thermo-mechanical degradation of the TBCs due to CMAS infiltration while maintaining the physical and mechanical properties of the TBC continues to be a challenge.
Accordingly, it is desirable to provide engine structures and methods of forming the engine structures with a TBC that has a microstructure of columnar grains with gaps defined between the columnar grains, and with the TBC protected against CMAS infiltration into the gaps without sacrificing the physical and mechanical properties of the TBC. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.